In the field of silver halide photographic materials, methods for forming images by a diffusion transfer process are widely employed. The usefulness of the methods is also commonly recognized in view of instant formation of image.
In methods for forming color images by a diffusion transfer process, a so-called subtractive process in which color images are formed by a combination of yellow, magenta and cyan dye providing compounds is generally adopted. Among such methods, a color diffusion transfer photographic process has heretofore been well known using an azo dye image forming compound which provides an azo dye having a diffusibility different from that of the dye image forming compound as a result of development under a basic condition.
Examples of such image forming compounds releasing a cyan dye include those described in U.S. Pat. Nos. 3,942,987, 4,273,708 and 4,268,625.
However, the compounds as described in these references have been found to be disadvantageous because they have a nitro group in the para-position of the azo group which is subject to reduction during development that will cause discoloration. It has been also found that azo dyes containing a nitro group are generally subject to reduction by light, and thus they provide poor fastness of image to light.
In JP-A-53-66227 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), cyan azo dye forming compounds having a trifluoromethanesulfonyl group in the para-position of the azo group are described. However, such compounds are disadvantageous in that the incorporation of fluorine is troublesome from the standpoint of synthesis of the compound and environmental pollution. Moreover, further improvements on clearness of hue and transferability of dye released therefrom have been desired with these compounds.
Furthermore, cyan dyes having an acylamino group in the 2-position of a naphthol group are described in JP-A-63-40l52. While fastness of the dye is improved, solubility of the dye providing compound per se is sometime severely deteriorated by the introduction of the acylamino group into the 2-position of the naphthol.
In embodiments using the pre-formed dyes described above, there is an advantage that dyes which are excellent in hue can be utilized. In order to enjoy this advantage, the dyes are further requested to possess other many characteristics. Such requirements make difficult the development of dyes for diffusion transfer process. Examples of the desirable characteristics include (1) hue preferred in view of color reproduction, (2) dissociation property in the form of a low pKa necessary to be mordanted, (3) sufficiently high transferability to achieve a satisfactorily high maximum image density and to reduce the image forming time, (4) stability of hue and color image under mordanted condition against light, humidity, pH or temperature, and (5) superior aptitude for use as an image forming compound in a silver halide photographic material, such as solubility, synthesis aptitude or stability of a dye providing compound.
Although various compounds have been proposed as described in know references mentioned above as image providing compounds, particularly cyan dye providing compounds, only few of them satisfies the above described requirements. Further, in many cases, the desired hue of the dye varies, depending on the use of the photographic material or on the presence of other dyes employed in combination. Therefore, development of a novel coloring material which satisfies those requirements has been desired.
The inventors have been made investigations on naphthol azo dyes as cyan image forming dyes. As a result, it has been found that naphthol azo type cyan dye providing image forming compounds have the following two severe problems. The first problem is to achieve a coexistence of hue preferred in color reproduction with fastness of dye. More specifically, although a naphthol azo dye having a nitro group in the 4-position to the azo group has a good level of hue, it has insufficient fastness. A dye having no nitro group exhibits a good level of fastness, but a long wavelength range absorption is further requested, while the long wavelength range absorption has been realized to some extent as described in JP-A-63-40152. The second problem is to achieve a coexistence of the transferability of dye with a solubility of dye providing compound. More specifically, since a dye portion is designed to be hydrophilic in order to improve transferability, the solubility of the dye providing compound is generally contradictory to the transferability. Therefore, it is difficult to provide a coexistence between the transferability of dye and the solubility of image forming compound.